/**
 * JDBM LICENSE v1.00
 *
 * Redistribution and use of this software and associated documentation
 * ("Software"), with or without modification, are permitted provided
 * that the following conditions are met:
 *
 * 1. Redistributions of source code must retain copyright
 *    statements and notices.  Redistributions must also contain a
 *    copy of this document.
 *
 * 2. Redistributions in binary form must reproduce the
 *    above copyright notice, this list of conditions and the
 *    following disclaimer in the documentation and/or other
 *    materials provided with the distribution.
 *
 * 3. The name "JDBM" must not be used to endorse or promote
 *    products derived from this Software without prior written
 *    permission of Cees de Groot.  For written permission,
 *    please contact cg@cdegroot.com.
 *
 * 4. Products derived from this Software may not be called "JDBM"
 *    nor may "JDBM" appear in their names without prior written
 *    permission of Cees de Groot. 
 *
 * 5. Due credit should be given to the JDBM Project
 *    (http://jdbm.sourceforge.net/).
 *
 * THIS SOFTWARE IS PROVIDED BY THE JDBM PROJECT AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT
 * NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
 * FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL
 * CEES DE GROOT OR ANY CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * Copyright 2000 (C) Cees de Groot. All Rights Reserved.
 * Contributions are Copyright (C) 2000 by their associated contributors.
 *
 * $Id: FreeLogicalRowIdPage.java,v 1.1 2000/05/06 00:00:31 boisvert Exp $
 */

package com.sunchangming.stmdb.recman;

/**
 * Class describing a page that holds logical rowids that were freed. Note that
 * the methods have *physical* rowids in their signatures - this is because
 * logical and physical rowids are internally the same, only their external
 * representation (i.e. in the client API) differs.
 */
class FreeLogicalRowIdPage extends PageHeader {
	// offsets
	private static final short O_COUNT = PageHeader.SIZE; // short count
	static final short O_FREE = (short) (O_COUNT + Magic.SZ_SHORT);
	static final short ELEMS_PER_PAGE = (short) ((RecordFile.BLOCK_SIZE - O_FREE) / PhysicalRowId.SIZE);

	// slots we returned.
	final PhysicalRowId[] slots = new PhysicalRowId[ELEMS_PER_PAGE];

	/**
	 * Constructs a data page view from the indicated block.
	 */
	FreeLogicalRowIdPage(BlockIo block) {
		super(block);
	}

	/**
	 * Factory method to create or return a data page for the indicated block.
	 */
	static FreeLogicalRowIdPage getFreeLogicalRowIdPageView(BlockIo block) {

		final BlockView view = block.getView();
		if (view != null && view instanceof FreeLogicalRowIdPage)
			return (FreeLogicalRowIdPage) view;
		else
			return new FreeLogicalRowIdPage(block);
	}

	/** Returns the number of free rowids */
	int getCount() {
		return block.readInt(O_COUNT);
	}

	/** Sets the number of free rowids */
	private void setCount(short i) {
		block.writeInt(O_COUNT, i);
	}

	/** Frees a slot */
	void free(int slot) {
		get(slot).setBlock(0);
		setCount((short) (getCount() - 1));
	}

	/** Allocates a slot */
	PhysicalRowId alloc(int slot) {
		setCount((short) (getCount() + 1));
		get(slot).setBlock(-1);
		return get(slot);
	}

	/** Returns true if a slot is allocated */
	boolean isAllocated(int slot) {
		return get(slot).getBlock() > 0;
	}

	/** Returns true if a slot is free */
	boolean isFree(int slot) {
		return !isAllocated(slot);
	}

	/** Returns the value of the indicated slot */
	PhysicalRowId get(int slot) {
		if (slots[slot] == null)
			slots[slot] = new PhysicalRowId(block, slotToOffset(slot));
		;
		return slots[slot];
	}

	/** Converts slot to offset */
	private short slotToOffset(int slot) {
		return (short) (O_FREE + (slot * PhysicalRowId.SIZE));
	}

	/**
	 * Returns first free slot, -1 if no slots are available
	 */
	int getFirstFree() {
		for (int i = 0; i < ELEMS_PER_PAGE; i++) {
			if (isFree(i))
				return i;
		}
		return -1;
	}

	/**
	 * Returns first allocated slot, -1 if no slots are available.
	 */
	int getFirstAllocated() {
		for (int i = 0; i < ELEMS_PER_PAGE; i++) {
			if (isAllocated(i))
				return i;
		}
		return -1;
	}
}
